In many industrial processes using a gaseous feed stream is it is desirable or necessary to remove carbon dioxide from the gaseous feed stream prior to certain steps of the process. For example, in the separation of atmospheric air into its component parts by cryogenic distillation, it is necessary to prepurify the air by removal of carbon dioxide and water vapor from the air feed prior to refrigerating the air; otherwise, these gases would condense and freeze in the refrigeration heat exchange equipment and eventually clog the equipment, thereby necessitating removal of the equipment from service for removal of the frozen carbon dioxide and ice. The carbon dioxide and water vapor can be removed from the air by a number of techniques.
One well known method of removing carbon dioxide and water removal from gas streams is by the use of pairs of reversing heat exchangers that are operated alternately, such that one heat exchanger is in purification service while the other is undergoing frozen carbon dioxide and ice removal. Specifically, in this method the gas feed is passed through one heat exchanger in exchange with a refrigerant, which causes the carbon dioxide and water vapor to freeze onto the surfaces of the heat exchanger. When the buildup of frozen carbon dioxide and ice in the heat exchanger reaches a certain level, the heat exchanger is taken out of service to remove, by melting, the frozen carbon dioxide and ice. The other heat exchanger of the pair, from which frozen carbon dioxide and ice have been removed, is then placed into purification service. This method has the disadvantage that a considerable amount of regeneration gas is required to melt the frozen carbon dioxide and ice.
A popular method of removing carbon dioxide and water vapor from gas streams is adsorption. One common adsorption method use for air prepurification is PSA using two serially-connected adsorption beds, the first bed containing a desiccant, such as silica gel or activated alumina for water vapor removal, and the second bed containing a carbon dioxide-selective adsorbent, such as sodium-exchanged type X zeolite (13X zeolite). A two layer air prepurification system comprising a first bed of adsorbent selective for the removal of water from an air stream, for example alumina or silica gel, and a second bed of adsorbent selective for the removal of carbon dioxide, for example, 5A, 13X, calcium X or sodium mordenite, is disclosed in U.S. Pat. No. 4,711,645. Other two layer air prepurification PSA processes are described in U.S. Pat. Nos. 5,110,569 and 5,156,657, the disclosures of which are incorporated herein by reference. This method has a number of disadvantages. It is difficult to desorb carbon dioxide from the 13X zeolite, the zeolite develops "cold spots" in the upstream region of the bed of adsorbent and the adsorbent loses some of its adsorption capacity with time. TSA has also been practiced using this combination of beds. U.S. Pat. No. 5,110,569, mentioned above, shows such a process. A major disadvantage of the described TSA process is that a great quantity of heat energy is required in the adsorbent regeneration step, since both beds must be heated sufficiently to drive off the adsorbed moisture and carbon dioxide.
U.S. Pat. Nos. 4,249,915 and 4,472,178 disclose a two-step adsorption process for removing water vapor and carbon dioxide from atmospheric air comprising a first step in which water vapor is removed from the air by PSA using an adsorbent such as alumina, silica gel, 13X zeolite or 5A zeolite and a second step in which carbon dioxide is removed from the air by TSA using an adsorbent selective for carbon dioxide, such as 13X zeolite or 5A zeolite.
Air prepurification by PSA has also been practiced using a single bed of adsorbent which removes both water vapor and carbon dioxide. Such a process is disclosed in U.S. Pat. No. 5,232,474, the disclosure of which is incorporated herein by reference. The principal disadvantages of this method of air prepurification is that it is difficult to produce ultra high purity air (air containing less than 1 ppm carbon dioxide) efficiently by this method, and a high volume of purge gas is required to effect adequate adsorbent regeneration.
Methods of producing air containing very low levels of water vapor, carbon dioxide, carbon monoxide, hydrogen and hydrocarbons are continuously sought. The present invention provides a method which accomplishes this, and does so with low energy and capital expenditures